Add dns64 plugin (#3534)

* Add dns64 plugin

Add external plugin to core in-tree.
* Pull code from upstream: https://github.com/serverwentdown/dns64
* Update docs.

Signed-off-by: Ben Kochie <superq@gmail.com>

* Make dns64 consistent.

Signed-off-by: Ben Kochie <superq@gmail.com>

* Cleanup README

Signed-off-by: Ben Kochie <superq@gmail.com>

* Cleanup minor issues.

Signed-off-by: Ben Kochie <superq@gmail.com>

* Remove proxy method.

Signed-off-by: Ben Kochie <superq@gmail.com>

* dns64: big cleanup

* Make the code a bit more idiomatic
* Add tests
* use proper Upstream API

Signed-off-by: Casey Callendrello <c1@caseyc.net>
Signed-off-by: Ben Kochie <superq@gmail.com>

* A little more clenaup

* Fix some docs.
* Use the correct plugin register method.
* Cleanup some review items.

Signed-off-by: Ben Kochie <superq@gmail.com>

* Add metrics counter for DNS64 translations

Add a basic counter of how many DNS64 translations have been completed.

Signed-off-by: Ben Kochie <superq@gmail.com>

* Add DNSSEC bug link

Signed-off-by: Ben Kochie <superq@gmail.com>

* Test cleanup

Signed-off-by: Ben Kochie <superq@gmail.com>

* dns64: more test cleanup

Signed-off-by: Casey Callendrello <c1@caseyc.net>

Co-authored-by: Casey Callendrello <c1@caseyc.net>

plugin/dns64/README.md

@@ -0,0 +1,64 @@
+# dns64
+
+## Name
+
+*dns64* - enables DNS64 IPv6 transition mechanism.
+
+## Description
+
+From Wikipedia:
+
+> DNS64 describes a DNS server that when asked for a domain's AAAA records, but only finds
+> A records, synthesizes the AAAA records from the A records.
+
+The synthesis in only performed if the query came in via IPv6.
+
+See [RFC 6147](https://tools.ietf.org/html/rfc6147) for more information.
+
+## Syntax
+
+~~~
+dns64 [PREFIX] {
+  [translate\_all]
+}
+~~~
+
+* [PREFIX] defines a custom prefix instead of the default `64:ff9b::/96`
+* `translate_all` translates all queries, including respones that have AAAA results.
+
+## Examples
+
+Translate with the default well known prefix. Applies to all queries
+
+~~~
+dns64
+~~~
+
+Use a custom prefix
+
+~~~
+dns64 64:1337::/96
+# Or
+dns64 {
+    prefix 64:1337::/96
+}
+~~~
+
+Enable translation even if an existing AAAA record is present
+
+~~~
+dns64 {
+    translate_all
+}
+~~~
+
+* `prefix` specifies any local IPv6 prefix to use, instead of the well known prefix (64:ff9b::/96)
+
+## Bugs
+
+Not all features required by DNS64 are implemented, only basic AAAA synthesis.
+
+* Support "mapping of separate IPv4 ranges to separate IPv6 prefixes"
+* Resolve PTR records
+* Follow CNAME records
+* Make resolver DNSSEC aware. See: [RFC 6147 Section 3](https://tools.ietf.org/html/rfc6147#section-3)